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Greg is right in that you have the pressure switch
set too high for the configuration of your system. The split seam of
your last seal may also testify to this.
The air pump, like your engine, has a "compression ratio" which limits
the maximum pressure output to a multiple of the ambient pressure. The
compression ratio is the volume of the system with the piston at the
bottom of the stroke divided by the volume of the system with the
piston at the top of its stroke.
In Carl's case he needs to:
1) Find the leak. There IS a small leak or the pump would never need to
run. The leak could be across the check valve and back through the
pump. It could also be at the pneumatic switch.
2) Measure the maximum pressure output of the pump at sea level with
the pressure switch bypassed. Disconnect the door seal during this
test or you WILL burst it. Reconnect the pressure switch and adjust
the cutout pressure to not more than 1/4 of the maximum sea level
pressure. The ambient air pressure at FL280 is about 1/3 of sea level.
Check with the manufacturer for the recommended inflation pressure
range of the seal and be sure you are within those limits.
In my plane I added a pump inlet filter to the door seal system to
prevent dirt from getting into the check valves and causing leaks. For
a filter I used a bubble stone from an aquarium, the ones used to
aerate the tank. 9 years later, no leaks.
On the topic of pump compression ratio, in the case of the manual gear
extension pump the system volume includes
the volume of the lines to the check valves. This becomes and
important point when the system is full of air and in need priming. For
best performance the check valves must be mounted as close as practical
to the pump.
Imagine the extreme case where there was a mile of tubing between
the piston off the pump and the check valve. On an engine, this would
be like adding a 6 foot stack of head gaskets under the head. Your
compression ratio drops to near 1:1 because you have a relatively small
piston stroking inside a large volume. In this case size does matter.
Regards
Brent Regan
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